The present invention relates to phase-locked loops (PLL), and more specifically to controlling a wideband, high sensitivity voltage-controlled oscillator (VCO) within a PLL.
A Hybrid Fiber-Coax (HFC) system is a cable architecture for providing residential video, voice telephony, data, and other interactive services to subscribers"" homes over fiber optic and coaxial cables. HFC systems operate over radio-frequency (RF) carriers. With respect to the downstream transmission path, cable router upconverters at the cable system head-end perform the modulation function, and cable modem receivers at subscribers"" premises perform the demodulation function. One typical modulation scheme used in the downstream path is Quadrature Amplitude Modulation (QAM).
The Data Over Cable Service Interface Specification (DOCSIS) defines interface requirements for cable modems involved in high-speed data distribution over cable television networks. Cable modem receivers and cable router upconverters in HFC data networks are required to operate over a wide downstream frequency band of 88-860 MHZ. For both the cable modem and the router, a wideband phase-locked loop (PLL) is necessary to tune the approximately 800 MHZ wide band. The wideband PLLs must be capable of tuning the 800 MHZ band while maintaining the noise characteristics and stability necessary for 256 QAM demodulation and modulation, as well as meeting the stringent DOCSIS performance specifications.
With respect to the cable router upconverter, or cable modem termination system (CMTS) as referred to by DOCSIS, meeting the PLL design requirements for loop stability, phase noise performance, and the wide operating range has meant trading off between the available DC tuning range versus performance and stability. In order to meet the stringent DOCSIS CMTS phase noise specifications, current upconverter designs use a split band approach that employs multiple, narrower range upconverters that tune some smaller portion of the downstream frequency band. With this scheme the downstream frequency band is broken into two or more sections with each range of frequencies having its own module with a separate voltage-controlled oscillator (VCO).
The use of multiple VCO modules is undesirable. Specifically, in the competitive DOCSIS standard cable data market, it is important from a marketing standpoint that a single upconverter card cover the entire downstream frequency range. A single full-band card solution relieves manufacturers from having to build, track, and stock multiple modules. For the Multiple Service Operators (MSOs), a single card alleviates similar ordering and stocking difficulties and simplifies network management by eliminating the need to track upconverter type with downstream channel assignment. There is also an incomplete solution stigma associated with requiring multiple modules.
Wideband VCOs are presently available that will tune the entire frequency range and could allow for a single VCO module, full-band upconverter. A disadvantage with this approach, however, is that any VCO designed to cover the full 800 MHZ bandwidth generally requires both a large control voltage range and high tuning sensitivity (MHZ/Volt). Large control voltage requirements mean that PLL loop filter DC gain must be added. Increasing loop filter gain allows for supplying the large control voltage range, but the added gain degrades phase noise performance and decreases loop stability. As an alternative, the VCO sensitivity could be increased to allow for reduced loop filter gain, but this accomplishes little as the gain is not being reduced but is simply being moved to a different place in the loop, resulting in the same stability and phase noise problems.
Thus, there is a need for an apparatus and/or method for generating the necessary control voltage for a wideband VCO while adding minimal noise and maintaining stability of the loop.